After multiple uses and laundering processes, white cotton fabric items can often obtain a yellowed appearance or cast. The yellow is produced by the absorption by the used fabric of short wavelength light typically in the blue to violet to ultraviolet frequencies commonly about 400-550 nanometers (nm). The absorption of these bluish wavelengths from ambient light imparts a visible yellow tint. In order to restore the appearance of the fabric to a bright white appearance, optical brighteners are often used. Such brighteners absorb in the typically invisible, ultraviolet wavelengths of about 275 to 400 nm and then re-emit at wavelengths typically from about 400 to about 525 nm. The peak of the common emission curve of energy from optical brighteners is well in the blue range of visible spectrum and is typically from 425-450 nm. The emitted blue light masks the yellowish appearance in a complimentary fashion and results in a bright white appearance.
Optical brighteners have been common in laundry detergents. Ramachandran, U.S. Pat. No. 4,140,641 discloses a concentrated liquid detergent for fabric containing a variety of ingredients including detergent components, softener components and an optical brightener. Wicksen, U.S. Pat. No. 4,790,856 discloses a softener antistatic agent containing softening and antistatic ingredients in combination with a brightener.
Brighteners are also disclosed in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons (1985) at pp. 184-185.
In restaurants, fast food and other such environments, having large quantities of soil resulting in fabric that resists brightening, simple laundering of fabric items in well formulated brightened laundry detergents can result in a less than bright white finish. One particular longfelt need is in the laundering of white cotton fabric towels or white cotton terry cloth towels in institutional or commercial kitchen environments. Such towels come in contact with substantial quantities of difficult to clean fatty soils which can have a variety of interactions with inorganic soils, oxidizing atmospheres, and other conditions resulting in hard to clean and yellowed fabrics. While the use of brightening agents is known and brightening agents have been formulated in both laundry detergents and softening agents, substantial need exists for improving laundry processes for cleaning soiled such white cotton fabrics. Many cotton fabrics such as white terry cloth towels become so soiled or stained that a simple laundry process cannot remove staining and/or obtain the desired bright white appearance.
Presoak compositions are also known for use in laundry processes in which the soiled material is contacted with a prespot or prestain composition or soaked in a presoak composition for the purpose of bringing soil removing compositions in intimate contact with the soil or stain. Presoak and prestain materials are used in both household and commercial or institutional laundry processes to remove stubborn soil such as grass stains, blood stains, food soils, soil from shop rags, kitchen cleaning operations, etc. Such presoaks primarily rely on the detergent activity of anionic and nonionic surfactant materials to initiate a soil/surfactant interaction that begins the removal process of soil from fabric. When the prespotted or presoaked material is then laundered, the pretreated soil becomes more easily removable. We have found that even through the use of conventional prespotting and presoaking compositions that when cleaning white cotton fabrics contaminated with certain soils, the fabrics do not achieve the desired bright white appearance.